1. Technical Field
The present invention is directed to pressure relief valves, and more particularly to pressure relief valves for medical applications using positive pressure pumps such as roller pumps.
2. Background Information
Many medical procedures utilize positive pressure pumps to move fluids to and from a patient. For example, cardiopulmonary bypass procedures require blood be moved from a patient to and through an external extracorporeal oxygenation circuit. During open heart surgery, it is necessary to deliver cardioplegia solution in order to maintain a patient's heart in a state of diastolic arrest. Kidney patients requiring dialysis treatments transfer blood through a dialysis machine for removal of toxins.
It has been a longstanding practice to utilize a roller pump to move fluids in medical applications. A roller pump uses a peristaltic action to advance fluids by moving a rotor in a rolling motion across the surface of tubing. The rolling motion of the rotor compresses the tubing against a support member, thereby pushing fluid ahead of the rotor. One advantage of a roller pump is the gentleness with which it pumps fluid through a length of tubing; this is particularly important when pumping fluids containing blood since less gentle treatment can result in lysis of the blood cells. Another advantage of roller pumps is that the fluid never touches the pump, permitting the pump to be reused by simply replacing the tubing for each new medical procedure.
Roller pumps are not without disadvantages. One problem arises from the ability of the roller pump to generate substantial pumping pressure. On occasion, a circuit through which fluid is desired to be pumped may become clogged, or more commonly it is simply closed off during setup and inadvertently left closed off when pumping is commenced. Typically, for example, a fluid circuit is primed with fluid so as to remove all air bubbles, and then closed off until it is time to pump the fluid. It occasionally happens that a medical team employing a roller pump will forget to open the appropriate valves before commencing operation of the roller pump. The high pressure generated by a roller pump can result in damage to system components. It can even result in an explosive separation of tubing from one or more components, resulting in an uncontrolled spraying of fluid contained in the tubing around the surgical area. If the closed valve is opened while excessive pressure exists, a high pressure flow can be delivered into the patient, potentially causing serious injury. If the tubing contains an obstruction, that obstruction may give way under increased pressure, and may be introduced into the patient, also potentially causing serious injury.
Because of these disadvantages of roller pumps, some medical practitioners have given up the significant advantages of roller pumps for other types of pumps that generate lower pressures. For example, centrifugal pumps are currently in wide use for the same uses as roller pumps because the pressures they will generate if blocked become self-limiting at pressures significantly less than those generated by roller pumps. Centrifugal pumps are subject to some of the same problems as roller pumps, however, such as generating higher than normal pressures when a blockage occurs, even though they are less likely than roller pumps to result in injury and are unlikely to cause explosive failure of the system components. Centrifugal pumps suffer from the significant disadvantage that they are more expensive to use than roller pumps. Since they operate by moving fluid directly through the pump, they must be replaced after a single use. By way of contrast, a roller pump may be reused indefinitely; only the associated tubing requires replacement after each procedure.
Various solutions have been sought to solve disadvantages such as those mentioned above. U.S. Pat. No. 5,052,900 (hereinafter sometimes referred to as the '900 patent), the disclosure of which is incorporated herein by reference, describes the introduction of a pressure relief valve at a location between the outlet and inlet ports to a positive pressure pump. This valve is shown as being formed of a length of tubing which is closed off by a pair of tubular members biased toward one another by elastic bands with sufficient force to seal the tube. Application of sufficient pressure will overcome the bias of the elastic bands, permitting fluid from the outlet port of the pump to recirculate through the pump, thereby stopping any further pressure increase within the system. Use of this type of valve is beneficial in concept because it is inexpensive to make, allowing its disposal after each use without adding unreasonable cost to the medical procedure.
It has been discovered that this approach is not fully functional in practice. For example, it has been found difficult to construct a valve according to the teachings of the '900 patent that has a strong enough elastic bias member to close the valve tubing, yet will allow that same tubing to open up before substantial pressures build up. Thus, it only partially solves the problem of a pressure increase within the system. This problem appears to be severely exacerbated when the valve has been stored for some period of time, since the tubing tends to adapt to its compressed condition, requiring even greater pressure before it will open. Hence, this type of valve has been a disappointment to many medical practitioners.